# Using lenses

In this part of the tutorial, we cover the use of lenses with Ontop. Lenses allow us to create "virtual views" of specific operations over the data, that can be referenced in our mapping definitions for a more structured access to the database.

We will go over all lenses currently supported by Ontop:

  1. Basic Lens
  2. Join Lens
  3. Union Lens
  4. Flatten Lens
  5. SQL Lens


In this tutorial, we use a different database than in the previous sections. Before running any of the examples, please follow the setup instructions.

Not supported by Ontop-Protégé

Lenses are not supported by the Ontop plugin for Protégé. To follow this tutorial, please use a text editor.

# General

Lenses are provided to Ontop as a JSON file of the following structure:

    "relations": [Lens]

where each Lens object corresponds to one particular lens, holding a name through which it can be referenced in the mapping and a type which indicates the specific type of the lens. We can include any number of lenses in this file, and they can reference database relations as well as each other.

Some more common features of lenses will be discussed in the Basic Lens section.

In each of these sections, we will construct one mapping file and one lenses file that can be passed to Ontop. To simplify testing, a common template for all mapping files is already included in the tutorial files. You can continue the preparation of the individual mappings from this template.